When I paint in Photoshop, I blend colors using variable opacity under the control of a pressure-sensitive Wacom pen.
Grad-based blending from one color to another is not true digital color mixing, however. A linear blend from yellow to blue, for example, goes through middle gray without producing any sense of green.
If you want to simulate color mixing with opacity falloff blending from one color to another, you might want to help the blend along by adding a third color along the way.
Color mixing (which has the unfortunate and misleading name subtractive color mixing) is closer in behavior to multiplication, or the sandwiching of color filters. A yellow filter that only lets through pure yellow light (a single wavelength at around 570 nanometers) sandwiched with a blue (475 nm) filter will not create a green color, because neither filter allows green (510 nm) light to pass. It's the same deal with pigments.
A "sloppy" blue filter -- one that looks blue to the eye but lets other wavelengths slip through, sandwiched with a sloppy yellow filter, will produce a nice green, provided that there is plenty of green in each.
Real world pigments are like sloppy filters.
If real world paints could be manufactured with their reflective qualities confined to extremely narrow wavelengths, then they would look saturated and colorful but mix to produce black.
There is no exact mapping of RGB values to wavelengths. You could make conversion lookup tables based on the feelings of panels of human observers. For any given color as perceived by a person, there are many "wavelength recipes" that could be responsible for creating in that individual that particular color impression. How colors mix depends on what wavelengths are present in their wavelength recipe. When two colors mix, those wavelengths which both colors have in common survive the mix.
Multispectral sampling to represent colors with more than the usual 3 (red, green, and blue) is not a new idea in computer graphics. The ability to represent colors with arbitrarily many spectral samples (often 9 or 12 in practice) has been part of -- for example -- the Renderman Interface Specifications -- for some time now (see The Renderman Companion  by Steve Upstill, page 42, for discussion of this topic). I'd like to think this idea has worked its way into Painter IX's color mixing by now, but I haven't seen any specific discussion to this efect.
If you'd like a good handle on color mixing, don't miss Blue and Yellow Don't Make Green by Michael Wilcox.